The present invention relates to a photometric cell and is applied to the measurement of the optical density of fluids (for example liquid) and in particular to the measurement of the turbidity of river water or nuclear reactor water or to the detection of pollutants in liquids or gases.
Photometers are known which comprise a vessel containing the fluid to be analysed and optical means for passing a beam of light through the said vessel. The measurement of the attenuation due to the traversal of the vessel makes it possible to calculate the optical density of the fluid and the concentration of one of the substances present in the fluid.
Although such devices are suitable in certain cases they have the disadvantage of having a limited optical path for the light beam which limits their accuracy. Therefore devices having two generally spherical mirrors have been proposed, said mirrors being arranged on either side of the vessel in such a way that the light beam can pass to and fro between the mirrors and the optical path is lengthened for the same overall dimensions.
The photometric cell closest to that of the present invention is the so-called White cell, described more particularly in the Article by J. U. White, published in the J.O.S.A. Journal, Vol. 32, p 285 May 1942 and entitled "Long Optical Paths of Large Aperture."
Such a cell is diagrammatically shown in FIG. 1 and comprises two juxtaposed spherical half-mirrors M.sub.1 and M.sub.2 and a third spherical mirror M facing mirrors M.sub.1 and M.sub.2. The three mirrors have the same radius of curvature. Mirrors M.sub.1 and M.sub.2 have their centre of curvature C.sub.1 and C.sub.2 located on mirror M and slightly staggered relative to one another. The centre of curvature C of mirror M is disposed between the two half-mirrors M.sub.1 and M.sub.2. The light beam used for performing the measurement penetrates the cell by means of an entrance diaphragm E and leaves the cell after multiple reflections on the mirrors by an exit diaphragm S, said two diaphragms being disposed in openings made in the mirror M.
The operating principle of this device is illustrated by the diagram of FIG. 2 which shows mirror M viewed from the front and the images formed with it. On the basis of the Standard Laws of Optics it is known that the image of an object point located in the vicinity of the centre of curvature of a spherical mirror is an image point such that the centre of curvature, as a first approximation, is the centre of the segment formed by the object point and the image point. Thus, the entrance diaphragm E considered as an object I.sub.0 gives, after a first reflection in the half-mirror M.sub.1 a symmetrical image I.sub.1 of I.sub.0 with respect to the centre C.sub.1. In turn this image I.sub.1, by reflection on the second half-mirror M.sub.2 gives a second symmetrical image I.sub.2 of I.sub.1 with respect to the centre of curvature C.sub.2 of the second half-mirror and so on. Thus, as a result of multiple reflections on the mirrors a sequence of images is obtained, all of which differ from one another and substantially located in the plane defined by the centres of curvature and the entrance diaphragm, i.e. on mirror M. These images are all aligned on two straight lines parallel to the line joining the centres of curvature C.sub.1 and C.sub.2. The exit diaphragm S is positioned in such a way that it coincides with one of these images (with the eleventh image I.sub.11 in FIG. 2). In other words the entrance and exit diaphragms are optically conjugated by the mirrors after a large number of reflections.
It is clear that such a device leads to a relatively large optical path because the number of reflections y is high (in the example of FIG. 2 the light beam traverses the cell 22 times during the 11 to and fro movements).